Operating System Basics · Topic 1 of 1

Operating Systems and Basic Linux Operations

Learn OS functions, kernel and shell, Linux distributions such as openEuler, the directory tree, file and user commands, chmod permissions and vi modes.

5 min

In a nutshell

The kernel manages hardware and resources; the shell interprets user commands.
Linux has one tree from /; configuration lives in /etc, logs in /var/log, root's home is /root.
r = 4, w = 2, x = 1, in the order owner, group, others.

What it is

An operating system (OS) is the software layer between the hardware and applications. It manages CPU time, memory, storage and devices for programs and users.

Linux is the OS most cloud platforms run on. Virtualization hosts and most cloud VMs use it, so basic Linux skills are essential.

How it works

What an OS does

Function	What it means
Process management	Creates, schedules and ends processes so many programs share the CPU
Memory management	Allocates memory to processes and isolates them from each other
File management	Organizes data into files and directories on storage
Device management	Drives hardware through device drivers
User interface	Offers a command line (shell) or a graphical desktop

The kernel is the core that does this work and talks to the hardware. The shell is the command interpreter that reads what the user types and asks the kernel to do it.

Bash is the most common Linux shell. Common OS families are Windows, UNIX, macOS and Linux.

Linux and its distributions

Linus Torvalds released the first Linux kernel in 1991 as open source under the GPL. A distribution bundles the kernel with tools, libraries and a package manager.

Distribution	Notes
Red Hat Enterprise Linux (RHEL)	Commercial, RPM packages, widely used in enterprises
CentOS	Community distribution derived from RHEL; today CentOS Stream sits just upstream of RHEL
SUSE Linux Enterprise	Commercial, RPM packages
Ubuntu, Debian	DEB packages, popular on cloud VMs
openEuler	Open source distribution started by Huawei, supports x86 and ARM (Kunpeng)
EulerOS	Huawei's commercial server OS

The directory tree

Linux has one tree that starts at the root, /. Disks and partitions are mounted somewhere inside it, not given drive letters.

Directory	Contents
/bin, /sbin	Essential user and system commands
/boot	Kernel and boot loader files
/dev	Device files, such as disks
/etc	System configuration files
/home	Home directories of normal users
/root	Home directory of the root user
/tmp	Temporary files
/usr	Installed programs and libraries
/var	Changing data such as logs in /var/log
/proc	Virtual files showing kernel and process information

An absolute path starts at /, such as /etc/hosts. A relative path starts from the current directory: . is the current directory, .. the parent and ~ the user's home. The prompt ends in # for root and $ for a normal user.

Common commands

Task	Commands
Where am I, what is here	`pwd`, `ls -l`, `ls -a` (shows hidden files)
Move around	`cd /var/log`, `cd ..`, `cd ~`
Create and remove	`mkdir -p a/b`, `touch file`, `rm file`, `rm -r dir`, `rmdir dir`
Copy and move	`cp file1 file2`, `cp -r dir1 dir2`, `mv old new` (also renames)
View files	`cat`, `more`, `less`, `head -n 20`, `tail -f /var/log/messages`
Search	`find / -name hosts`, `grep error file`
Archive	`tar -czvf backup.tar.gz dir`, `tar -xzvf backup.tar.gz`
Help	`man ls`, `ls --help`

Users and permissions

useradd user1 creates a user, passwd user1 sets the password, and su - user1 switches to it. User accounts are listed in /etc/passwd and password hashes in /etc/shadow.

ls -l shows permissions as ten characters, such as -rwxr-xr--. The first is the file type (- file, d directory), then three groups of rwx for the owner, the group and others. In numbers, r is 4, w is 2 and x is 1.

chmod 754 script.sh        # owner rwx, group r-x, others r--
chown user1:group1 file    # change owner and group

Network and services

Task	Commands
Show addresses	`ip addr`, or the older `ifconfig`
Test reachability	`ping 192.168.1.1`
Show listening ports	`ss -tlnp`
Manage services	`systemctl status sshd`, `systemctl start`, `stop`, `restart`, `enable`
Shut down or restart	`shutdown -h now`, `reboot`

On RHEL family systems such as CentOS and openEuler, interface settings are traditionally kept in files such as /etc/sysconfig/network-scripts/ifcfg-eth0.

The vi editor

vi (and its improved version vim) has three modes.

Last line command	Effect
`:w`	Save
`:q`	Quit if nothing changed
`:wq` or `:x`	Save and quit
`:q!`	Quit without saving

A worked example

An engineer must change a host's name server setting. They run cd /etc, check with pwd, back up the file with cp resolv.conf resolv.conf.bak, and open it with vi resolv.conf.

They press i, edit the line, press Esc and type :wq. Then they check it with cat.

How the exam asks about this

"Which command shows the current working directory?" → pwd.
"Which permissions does chmod 640 give?" → Owner read and write, group read, others nothing.
"Which vi command quits without saving changes?" → :q!.

What to remember for the exam

The kernel manages hardware and resources; the shell interprets user commands.
Linux has one tree from /; configuration lives in /etc, logs in /var/log, root's home is /root.
r = 4, w = 2, x = 1, in the order owner, group, others.
systemctl manages services; ip addr shows addresses; tar -czvf creates a compressed archive.
vi has command, insert and last line modes; :wq saves and quits, :q! quits without saving.

Watch out

rm -r deletes a directory tree without a recycle bin. Check the path with pwd and ls before running it, especially as root.

Common mistakes

Confusing /root and /. / is the top of the whole tree. /root is only the root user's home directory.

Reading permissions in the wrong order. The three groups are always owner, group, others. In -rw-r----- only the owner can write.

Thinking mv only moves files. mv also renames a file or directory when the target is in the same place.

Check yourself

What is the difference between the kernel and the shell?
The kernel manages the hardware, processes and memory. The shell reads the user's commands and passes the work to the kernel.
Which directory holds most system configuration files?
/etc. Logs are under /var/log and user homes under /home.
What does chmod 755 file set?
Read, write and execute for the owner, and read and execute for the group and for others.

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